1. Technical Field of the Invention
The present invention relates to a transflective liquid crystal display device in which a display screen is illuminated from back side in a dark place and a display screen is illuminated by utilizing ambient light in a light place, and a light source device and a member suitable for this.
More particularly, the present invention relates to a transflective film and a transflective polarizer, which can enhance utilization efficiency of light to make a screen brighter or can elongate the usable time of a battery, and a polarizing light source device and a transflective liquid crystal display device using them.
2. Description of the Related Art
Liquid crystal display devices are used in various fields since they are portable and light. A liquid crystal molecule in a liquid crystal display device is not a light emitting substance such as used in a cathode ray tube (CRT) and the like and only has a function as a light valve simply controlling polarization condition of light, therefore, a display screen of a liquid crystal display device is dark and invisible unless illuminated by some means. Namely, a reflective liquid crystal display device adopts a method by which ambient light is incorporated into a liquid crystal display device and a display screen is illuminated by this light.
However, in a reflective liquid crystal display device utilizing exclusively ambient light for illumination of a display screen, though excellent visibility is obtained outdoor under sunshine, since ambient light is weak in dark place such as night environment and the like, a display screen cannot be illuminated sufficiently to give a dark screen, decreasing visibility remarkably.
As a result, widely adopted are methods in which a liquid crystal display device is not of complete reflective mode and a display screen is illuminated using an auxiliary light source in a dark place, namely, is also of transmissive mode.
This dual function of reflection and transmission leads to the designation, “transflective”. Therefore, a liquid crystal display device having dual mode of reflective mode and transmissive mode is called a transflective liquid crystal display device.
Here, a conventional transflective liquid crystal display device is described using FIG. 17. Generally, in a liquid crystal display device, by electrically changing the orientation condition of liquid crystal molecules enclosed in liquid crystal cell 30, the polarization condition of light passing through this is controlled, and the liquid crystal cell 30 is constituted of a pair of facing transparent electrodes, namely, a back side transparent electrode 31 and a front side transparent electrode 32, and a liquid crystal layer 33 sandwiched between them. Though omitted in the figure, the liquid crystal cell 30 also has, in addition to the above-mentioned components, cell substrates placed on both most outer surfaces, an orientated film for orienting the liquid crystal layer 33, and a color filter layer and the like in the case of color display.
At the front surface of the liquid crystal cell 30, an absorption-type polarizer 41 for detecting the polarization condition of light transmitting it is placed, and additionally, optical elements such as a phase retardar 42 and the like are also placed. On the other hand, on the back surface of the liquid crystal cell 30, a polarizing light source device 93 for extracting only specific polarized light and emitting it toward the liquid crystal cell 30 is placed, if necessary via a phase retarder (not shown in figure) at the back surface side. In the polarizing light source device 93, a transflective polarizer 92 comprising an absorption-type polarizer 90 and an optical film 91 having a transflective function is placed at a position facing the liquid crystal cell 30, further, on the back face side, a light source device 61 is placed. The light source device 61 comprises a light guide plate 52 having a light source 51 at the side or lower position, and a reflector 53 at the back of the light guide plate 52, and when the light source 51 is placed at the side position, light from this is reflected by a reflecting mirror 54, and substantially all of this is guided to the light guide plate 52, further, emitted toward the transflective polarizer 92. The transflective liquid crystal display device 94 is constituted as described above. Therefore, the conventional transflective polarizer 92 has a structure in which the absorption-type polarizer 90 and the optical film 91 having a transflective function are laminated, as shown in FIG. 18.
As the conventional optical film 91 having a transflective function used in such a transflective liquid crystal display device, there are known a film obtained by dispersing a light diffusing substance into a transparent or semi-transparent resin film as described for example in JP-A-55-46707, a film obtained by dispersing pearl luster pigments uniformly in a transparent substance and utilizing reflection at the surface of pearl luster pigments as described for example in JP-A-55-84975, and the like. These films had a problem that since a reflecting ability is manifested by back diffusing, the condensing effect of a lens sheet used for improving the front luminance of a transmissive liquid crystal display device is usually lowered, consequently, the front luminance is not improved.
On the other hand, in a transmissive liquid crystal display device, a system improving luminance using a reflection-type polarizer has recently been adopted as described, for example, in JP-A-63-168626, 6-51339 and 6-324333 and WO95/27919. In this system, by allowing a reflection-type polarizer to intervene between a light guide plate, which is a light source in a transmissive liquid crystal display device, and an absorption-type polarizer placed on a back surface side of a liquid crystal cell (hereinafter, referred to a back side absorption-type polarizer), one of the polarization component of light emitted from a light source or a light guide plate is reflected to return it to the light source or the light guide plate before being absorbed by the back side absorption-type polarizer, to cause polarization change or depolarization, consequently, light is recycled.
However, even if such a system improving luminance is tried to be applied to the conventional transflective liquid crystal display device, since the optical film 91 having a transflective function intervenes between the back side absorption-type polarizer 90 and the light source device 61 or the light guide plate 52 as shown in FIG. 17 in the conventional transflective liquid crystal display device, when a reflection-type polarizer is placed on the light source device 61 or the light guide plate 52, polarization condition is broken at the optical film 91 and a sufficient effect cannot be manifested.